This invention relates to a method of preparing impermeable, polycrystalline Pb-.beta."-alumina bodies capable of having their electrical properties investigated, and suitable for use in devices such as ionic probes, for example, in PbO glass systems.
The invention relates more specifically to a method for preparing impermeable, polycrystalline samples of Pb-.beta."-alumina ceramic from Na-.beta."-alumina ceramic by ion exchange. The method permits Pb ions to be introduced into polycrystalline samples of material having the .beta."-alumina structure without causing cracking of the sample. Prior to applicant's invention, it was not possible to prepare single crystal Pb-.beta."-alumina from single crystal Na-.beta."-alumina because the samples always cracked, rendering them unusable. This cracking was due to resultant stresses caused by anisotropic lattice expansion of the crystal by replacement of the smaller Na ion by the larger Pb ion.
It is known from the prior art that divalent .beta."-alumina compositions can be prepared by ion exchange, (see "Divalent Cation Conductivity in .beta."-alumina"; B. Dunn, R. M. Ostrom, R. Seevers and G. C. Farrington; Solid State Ionics 5 (1981) 203-204 whose disclosure is incorporated herein by reference). In the method disclosed therein, single crystals of Na-.beta."-alumina were immersed in various molten salt baths to replace the entire sodium ion content with divalent ions. In the case of Pb++ exchange, a very rapid exchange was achieved with nearly complete replacement, i.e., 95%, in very short times at low temperatures of about 525.degree. C. However, as noted previously, due to the fact that the Pb ion is so much larger than the Na ion, e.g., 1.21 angstroms as compared to 0.95 angstrom, if this substitution method is applied to polycrystalline samples the resultant stresses due to lattice expansion result in both microscopic as well as macroscopic cracking of the polycrystalline sample rendering it unsuitable for its intended use as an ionic probe as well as for other applications.
Other divalent cations have been substituted for Na ions in the Na-.beta."-alumina structure. In addition to Pb these are Sr, Ca, Zn, Fe, Mn, Cu, Ag, Sn, Cd and Ba. These substitutions, however, have been carried out using single crystal Na-.beta."-alumina. (For a discussion of substitution in single crystals see Solid State Ionics, Vol. 7 (1982) 267-281, "Divalent Beta"-Aluminas: High Conductivity Solid Electrolytes for Divalent Cations"; G. C. Farrington and B. Dunn).
Additionally, divalent cations (Ca, Sr, Ba, Zn, Cd, Sn and Pb) have been substituted for Na ions in polycrystalline Na-.beta."-alumina, (see Solid State Ionics, Vol. 11 (1983) 125-132, "Preparation and Properties of Polycrystalline Divalent-Cation .beta."-Alumina"; E. E. Hellstrom and R. E. Benner); however, divalent cations larger than the Na ions cause the polycrystalline samples to crack.